The invention relates to an irradiation module for a measuring apparatus which is configured to measure a position change of a measuring object in at least one spatial direction in relation to a reference object by irradiating the irradiation module with measuring light. Furthermore, the invention relates to a measuring apparatus comprising this type of irradiation module and to a mask inspection tool and a projection exposure tool for microlithography, respectively comprising this type of measuring apparatus.
Michelson interferometers are often used to measure position changes of a measuring object with respect to a reference object. An example of this type of Michelson interferometer known from the prior art in the embodiment as a homodyne laser interferometer is shown in FIG. 12. A monochromatic frequency-stabilised laser 612 is used as the light source here. Typically, there is fastened to the measuring object a mirror, in this case in the form of a reflection prism 620, the movement of which with respect to a fixed polarising beam splitter 616 and a fixed reference mirror, in the present case in the form of a further reflection prism, is determined.
In comparison with the use of simple end mirrors as reflectors and the use of a non-polarising beam splitter, the advantage of using the reflection prism and the polarising beam splitter is that the effect of cyclical errors, e.g., power fluctuations and the effects of tilts in the reflectors, can be reduced, as described, e.g., in the document by P. L. M. Heydemann, “Determination and correction of quadrature fringe measurement errors in interferometers,” Applied Optics, Vol. 20, No. 19, October 1981, pages 3382-3384.
A further improvement of the measuring results can be achieved by means of the so-called heterodyne measuring principle wherein the output beam of the laser consists of two components with different polarisation directions and slightly different frequencies. This enables particularly elegant measurement of the displacement of the measuring reflector. A phase difference between the measuring and the reference signal, which is proportional to the displacement of the measuring reflector, is produced.
All of these set-ups are associated with the basic problem that the optical path lengths in the interferometer arms not only differ due to the movement of the optical components, but also due to changes in the refraction index of the air. In principle, refraction index fluctuations can be measured with high resolution. In practice, however, it is only possible with difficulty to take these measurements at the precise position of the optical beams within the interferometer. This would be necessary, however, for a precise correction of the measuring deviations which occur.
Underlying Object
It is an object of the invention to provide an irradiation module and a measuring apparatus of the type specified at the start by means of which the aforementioned problems are resolved, and in particular a highly accurate position measurement, the accuracy of which is not affected by refraction index fluctuations in the air, is made possible.